Sound-reproducing system



Dec. 16, 1930. v A. C. KELLER 1,784,839

SOUND RERRODUC I NG SYSTEM Filed June 30, 1928 k E D d 3 q MAGNET/C I'ZI F/5L0 &

//V l/E N 70/? I ARTHUR 6! KELLER Br ATTORNEY Patented Dec. 16, 1930 UNITED STATES PA NT OFFICE ARTHUR C. KELLER, OFTNEW YORK, N. Y., ASSIGNOR TO BELL TELEPHONE LABO- RATORIES, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK sounn-nnraonuome SYSTEM Application filed .l'une so, 1928. Serial no. 289,516.

This invention relates to sound reproducing systems and more particularly to methods of improving the quality of reproduction in such systems.

During intervals when no recorded sounds are being heard such as at the start of 9. record or at other times during reproduction, the noise due to record irregularities is often very objectionable, particularly in systems employing high amplification. It is therefore the object of this invention to automatically eliminate this surface noise.

For this purposea sensitive relay device may be used in the reproducer circuit to short circuit the receiver or otherwise divert the reproduced currents whenever they fall to the noise level. Since the too sudden action of an ordinary relay causes an objectionable shock to the listener a special magnetron is provided in the receiver circuit to make this actionas gradual as desired. The proper characteristic for the magnetronmay be obtained in any well known manner such as'by locating the filament eccentrically with respect to the anode.

In the drawing Fig. 1 shows a reproducing system embodying this invention and Fig. 2 shows the normal characteristic of the sensitive relay device in Fig. 1 and also how this characteristic may be modified to suit the purpose of this invention.

1n the system shown in Fig. 1 the reproducer 1 is arranged to generate electric currents corresponding to the sounds recorded upon the record 2: These currents may be amplified to any desired extent by the amplifier 3 and may be utilized to operate ,loud

.speaker 4 over an'obvious circuit including transformers 5 and 6. The sensitive relay device 7 is preferably a magnetron of the general type disclosed in Patent 1,387 ,985, granted to A. W. Hull, August 16, 1921.

It is well known in the art that the current which flows through such a device is not affected by a magentic field weaker than a certain critical value but falls to zero'if the field is increased beyond this value. In the present invention advantage is taken of this property of the magnetron to prevent the loud speaker 4 from reproducing undesired sounds such as those due to the surface noise of the record 2. The rectifier 8 rectifies a portion of the current variations delivered by the amplifier and delivers the corresponding direct current to the magnetizing winding 9 of the magnetron. A polarizing winding 10 energized by the battery 11 is provided and may be used where necessary to supplement the magentic field produced by the main magnetizing winding.

During periods such as at the start of reproduction when the only currents generated by the reproducer 1 are those due to surface noise, the magnetizing current in the wind ing 9 will be very small. Consequently, the impedance between the circular anode 12 and the cathode 13 will be low as compared with the impedance of transformer 6 so that these currents which would otherwise produce objectionable noise in the loud speaker, are in this case diverted through the electron discharge device. When, however, theactual reproduction of speech or music begins the currents delivered b the amplifier are very much larger than efore and the magnetizing current of the winding 9 is correspondingly increased. By properly adjusting the output. of the rectifier and of the current in winding 10 if necessary, the total magnetic field in the magnetron may be made hi her than the critical value and the impe dance of the device 7 then becomes" almost infinite. Under this condition the anode current falls suddenly to zero as indicated by the curve 14, of Fig. 2. Transformer 6 now offers the lower impedance path to the currents representing the speech or music and the corresponding sounds are therefore reproduced by the loud speaker.

Since the magnetron in its ordinary form operates almost instantaneously, the transition from the condition of sound to silence, or vice versa, is very sudden and causes an objectionable shock to the listener. The action of the magnetron may be made more gradual in any one of various ways which are well known in the art. One such method would be to locate the cathode eccentrically with respect'to the anode as shown in the drawtion the impedance of the magnetron device may be made to vary with the strength of the magnetic field as shown by curve 15 of Fig. 2. As indicated by this curve the time of cut ofi in terms of change in field strength may be increased in this way from value a--t0 value b so-that no disagreeable sensation is ex erienced.

at is claimed is:'

1, In a sound reproducing system, a sound record, a current reproducing devlce coopcrating with said record, a loud speaker. responsive to the currents generated by said device, a magnetron connected between said device and said loud speaker, and meansfor rectifying a portion of the o'utputof said de: vice for exciting said magnetron. I

2. In a sound reproducing system, a sound record, a current reproducing device cooperating with the record,- a loud speaker responsive to the currents generated by the device, a magnetron adapted to vary in impedance gradually in response to changes in its excitation connected between the device and the loud speaker,- and means for rectifying a portion of the output of the device for excitmg the magnetron. 5 3, In aisound reproducing system, a sound record, a: current reproducing device coopcrating with the record,'a loud s eaker responsive to the currents generated y the device, a magnetron connected between the devic'e and-the loud speaker, and means for varying the impedance of the magnetron in accordance with the output of the reproducing device. g In witness whereof I hereunto subscrlbe my name this 25th day of June, 1928.

ARTHUR o. KELLER. 1 

